In Eukaryotic Cells, We Sometimes Find Reverse Transcriptase Activity, Even

In eukaryotic cells, we sometimes find reverse transcriptase activity, even though the cells are not infected with virus. This activity can be attributed to

A) infection with small bacteria known as mycoplasmas.
B) a novel prion infection.
C) the presence of viroids.
D) the presence of transposons integrating into polymerase genes.
E) the presence of retrotransposons.